Studies on Pathogenesis of Waardenburg Syndrome Type II and Tietz Syndrome Resulting from MITF Gene Mutations

Abstract

Microphthalmia-associated transcription factor (MITF) controls melanocyte survival and differentiation through directly regulating the expression of the tyrosinase(TYR)and tyrosinase-related proteins 1 and 2(TYRP1 and TYRP2)genes.MITFmutations have been reported to result in an abnormal melanocyte development and lead to Waardenburg syndrome type 2 (WS2), characterized by variable degrees of sensorineural hearing loss and patchy regional distribution of hypopigmentation. Recently,MITFwas also indicated as a causative gene for a more severe syndrome, the Tietz Syndrome (TS), characterized by generalized hypopigmentation and complete hearing loss. However, few functional studies have been performed to compare the diseases-causing mutations. Here, we analyzed thein vitroactivity of two recent identified WS2-associated mutation (p.R217I and p.T192fsX18) and one TS-associated mutation p.N210K. The R217I MITF retained partial activity, normal DNA-binding ability and nuclear distribution, whereas the T192fsX18MITFfailed to activateTYRpromoter due to loss of DNA-binding activity, and aberrant subcellular localization. The aberrant subcellular localization of T192fsX18 MITF may be caused by deletion of a putative nuclear localization signal (NLS) at aa 213-218 (ERRRRF). Indeed, MITF with deletion of the NLS fragment failed to translocate into the nucleus and activated theTYRpromoter. Tagging this NLS to GFP promoted the green fluorescence protein (GFP) translocated into the nucleus. The surprising finding of our study is that a TS-associatedMITFmutation, N210K, showed comparablein vitroactivity as WT. Thus, the possible involvement ofMITFin TS and its underlying mechanisms still need further investigation.